The concept of high-altitude, long-endurance solar-powered aircraft has been demonstrated by a number of air vehicle research projects in the past.
An example of previously built and flown state of the art is the AeroVironment aircraft, culminating in the Helios. Much of this is described in U.S. Pat. No. 5,804,284, to Hibbs, et al. (hereinafter, the Hibbs patent). The Hibbs patent shows a very large wingspan aircraft, with the solar collection and other mass distributed along a very high aspect ratio wing. This allowed the use of a very light wing spar, and the simple, clean design consumed very low power during the night. As can be appreciated, night time power usage is especially critical because the storage system is quite heavy. This means a large amount of solar energy must be collected to provide even a small amount of power at night. In the example given in the Hibbs patent, 2.5 Watt hours of electrical power had to be collected during the day to provide 1 Watt hour at night. More recently, the Solar Impulse II, a Swiss long-range experimental solar-powered fixed-wing aircraft, endeavored to achieve the first circumnavigation of the Earth. The Solar Impulse II provided 17,248 photovoltaic cells cover the top of the wings, fuselage, and empennage for a total area of 269.5 square meters (rated at 66 kW peak). The Solar Impulse employed four electric motors powered from the solar panels and four 41 kWh lithium-ion batteries, providing 13 kW, electric motors (17.4 HP) each.
Existing solar power systems typically rely on a solar array interfacing to a grid or battery system through a maximum power point tracker having a circuit assembly that adjusts the load impedance presented to the solar array in or to get the maximum power out of the array. These trackers, however, are heavy, costly, and detract from the total output power of the solar array. Moreover, typical large scale battery systems consist of a parallel strings of series connected battery cells that operate at the same voltage. Existing systems do not offer pack level monitoring and scale control sufficient for control of voltage controllable packs. In addition, the packs are typically connected in parallel through a busbar and/or wire harness, which results in increased weight and complexity. Also, typical systems cannot control the various parallel strings output voltage and are unable to balance the source current with secondary elements. Finally, existing packaging battery cell groups detract from the overall pack specific energy density and are therefore not designed to support long endurance aircraft while keeping rate production in mind. Thus, a need exists for a solar power systems that can overcome the deficiencies of the prior art, by providing improved control of the battery packs and solar panels. Such solar power systems may be employed with solar-powered aircraft, such as long endurance solar-powered aircraft.